Master pages may include placeholders %1 to %9 which are substituted at runtime with up to nine parameters. You can set these with leaf.Use as the report is defined, or you can update them as you go (for example to provide a ‘thumb-index’ for a workspace lister):

1 leaf.Subst flist[ct]   © current function shows in header

This will take effect at the next page throw. If you intend to manage your own page throws, substitute the parameters before you call leaf.NewPage to throw the page.